Jennifer Garner is a mom three times over as of last February, when her son Samuel was born, joining sisters Violet, 6, and Seraphina, 3. But she’s still most at home playing women who don’t have a clue about motherhood. “NOBODY is prepared for this,” she says with a laugh. “And sometimes, the harder you try, the more you mess up. There’s nothing that will humble you more. I am constantly feeling that I’ve screwed something up, or will screw something up. You learn as you go along.” Learning to be a mom has had fringe benefits for her as an actress. Garner earned some of the best notices of her career when she played a woman who can’t have a child in “Juno,” work that Entertainment Weekly called Oscar-worthy and “unbelievably touching.” In her new film, “The Odd Life of Timothy Green,” Garner plays another woman in search of a baby she cannot have. And this time, she and her husband (played by Joel Edgerton) find a little boy quite literally in the cabbage patch. Of course, Garner, 40, and husband Ben Affleck have access to all the child-rearing help Hollywood royalty can afford. She took 16 months off before and after Samuel’s birth. And on returning to work, she found the “mothering gene” still switched on as Cindy, mom-in-training to a magical little boy in “Timothy Green,” which opens Wednesday. CJ Adams has the title role in the film, a boy of 10 who springs up out of the garden to fulfill his parents’ longing, their hopes for a child and what he would be like. Garner gained fame as a butt-kicking TV secret agent (“Alias”) for J.J. Abrams, and played her share of society types (“Arthur”) and masked vigilantes (“Elektra”), but she still thinks of herself as a “small-town girl” from Charleston, W.Va.